1. Field of the Invention
The present invention relates to an oblique movement preventing device for an endless belt and an image forming apparatus with it.
2. Description of the Related Art
An electrophotographic type image forming apparatus is widely known. An image forming apparatus of this system includes an image forming unit that forms a desired image and a recording medium directly conveying device that directly conveys a recording medium (for example, a paper sheet) to the image forming unit. Then, the desired image formed in the image forming unit is directly transferred to the recording medium directly conveyed from the recording medium directly conveying device to the image forming unit. Alternatively, the image forming apparatus of this system includes a combination of an image holding device and a recording medium indirectly conveying device in place of the recording medium directly conveying device. In this alternative image forming apparatus, the desired image formed in the image forming unit is firstly transferred to the image holding device and then the desired image is transferred from the image holding device to the recording medium conveyed to the image holding device by the recording medium indirectly conveying device.
The image forming unit includes a photosensitive drum which rotates in a predetermined direction, an electrostatic charger, an exposure, a toner developer, a transfer, and a cleaner, which are successively arranged around the photosensitive drum in the predetermined rotating direction of the photosensitive drum.
The electrostatic charger uniformly charges the outer circumferential surface of the photosensitive drum that rotates in the predetermined direction. The exposure exposes the uniformly charged outer circumferential surface of the photosensitive drum to form an electrostatic latent image which corresponds to a desired image on the uniformly charged outer circumferential surface. The toner developer develops the electrostatic latent image on the outer circumferential surface of the photosensitive drum by using the toner. The transfer transfers the toner image on the outer circumferential surface of the photosensitive drum directly to the recording medium directly conveyed to the image forming unit by the recording medium directly conveying device or transfers it to the image holding device. In addition, the cleaner cleans the outer circumferential surface of the photosensitive drum after the toner image is transferred.
In an electrophotographic type image forming apparatus which forms a multicolor image, a recording medium directly conveying device which uses an endless belt or an image holding device which uses an endless belt are employed, and a plurality of image forming units that form image pieces of mutually different colors are arranged along the endless belt.
It is well known that the endless belt may cause oblique movement or meandering. In the event that oblique movement or meandering occurs in the endless belt, a position shift among the image pieces occurs in the multicolor image transferred from the plurality of image forming units to the recording medium conveyed by the endless belt of the recording medium directly conveying device or to the endless belt of the image holding device. Consequently, many techniques for preventing oblique movement of the endless belt have been conventionally proposed.
The most well-known conventional endless belt oblique movement preventing technique is to use at least one spindle-shaped crown roller in a plurality of supporting rollers which support the endless belt. However, in the endless belt used for a long time in combination with the crown roller, the length at the center portion becomes longer than that at each of the both side edge portion. And, the center portion of the endless belt warps as compared to the both side edge portions between the plural supporting rollers.
As a result, minute distortion is generated at a portion of the transferred multiple-color image on the recording medium conveyed by the endless belt of the recording medium directly conveying device or on the endless belt (so-called blanket) of the image holding device, that portion corresponding to the center portion of the endless belt.
In one of the conventional endless belt oblique movement prevention techniques, oblique movement regulating members are in contact with both side edges of the endless belt. However, in this endless belt oblique prevention technique, the both side edges of the endless belt are damaged in a comparatively short period by the sliding contact of the both side edges with the oblique movement regulating members. Reinforcing the both side edges of the endless belt slightly extends a life of the endless belt, but increases the manufacturing cost thereof.
In further one of the endless belt oblique movement preventing techniques, ribs that come in contact with both side ends of the endless belt supporting roller are mounted on the both the both side edges of the inner surface of the endless belt. However, in this endless belt oblique movement preventing technique, it is difficult to accurately mount the ribs at the predetermined positions of the both side edges of the inner surface of the endless belt. In addition, the ribs are easily damaged in a comparatively short period by bringing them in contact with the both ends of the endless belt supporting roller and increase the manufacturing cost of the endless belt.
JP-A 2002-2999 (KOKAI) discloses an endless belt oblique movement preventing device which has one swingably supported endless belt driving roller and an endless belt driving roller tilting mechanism. When the endless belt starts an oblique movement, the tilting mechanism tilts the endless belt driving roller to cancel the oblique movement of the endless belt. The swingably supported endless belt driving roller is applied with rotation force from a motor, which is a rotation driving source, by a rotation force transmitting mechanism. However, transmission of the rotation force by the rotation force transmitting mechanism becomes unstable when the endless belt driving roller is tilted. This means that transfer of the endless belt by the endless belt driving roller becomes unstable. As a result, a positional shift among image pieces and a distortion in a multicolor image transferred to the recording medium conveyed by the endless belt of the recording medium directly conveying device or to the endless belt of the image holding device.
To prevent transmission of rotation force from the rotation force transmitting mechanism from being unstable by the tilt of the endless belt driving roller, both the rotation force transmission mechanism and the motor should be configured to be tilted together with the endless belt driving roller. However, this configuration is complicated and increases the manufacturing cost of the endless belt oblique movement prevention device, and at the same time increases the outside dimensions of the device.